July 27, 2009 -- Obesity costs the U.S. health care system up to $147 billion a year: An extra $1,429 per year for each obese person.

It's not obesity itself that costs so much. It's the bad health that comes with it, says a new study.

"The medical costs attributable to obesity are almost entirely a result of costs generated from treating the diseases obesity promotes," lead study author Eric A. Finkelstein, PhD, director of North Carolina's RTI Public Health Economics Program, says in a news release.

If nobody in the U.S. were obese, we'd spend 9% less on health care. But more than a third of us are obese -- and another third of us are overweight.

That's a scary statistic. Here's a scarier one: 17% of U.S. children and teens are so overweight they're in the top 5% of body size for their age on growth charts. A less nice way to put it: these kids already are obese.

The health-related costs of obesity are rising. That's because more and more people are becoming obese, Finkelstein and colleagues calculate.